Error behaviour when session cannot be created due to "time cap" limitation

[jlurien]

Problem description

Use case would be that QoD is productized as number of N QoD minutes or sessions per period and after reaching that limit a new QoD session, or a session duration extension, is requested. What would be the right error to be answered? As an enhancement this can be added as example to document the expected behaviour.

Possible evolution
Add an explicit example to /paths/sessions/POST/responses/"429"/content/"application/json"/examples:

In the guidelines there is a Service Exception documented as:

Error Code	Description
TOO_MANY_REQUESTS	Either out of resource quota or reaching rate limiting.

SessionsTimeCapExceeded:
  summary: The requested duration exceeds the overall sessions time cap for this device in certain period of time or contract duration.
  value:
    status: 429
    code: TOO_MANY_REQUESTS
    message: The requested session exceeds the allocated quota for this device: Sessions time cap limit exceeded

Alternative solution
An alternative code may be created especifically for QUOTA_REACHED.

Additional context

Initially opened as discussion: #231

[jlurien]

I have decided to use status 429 but with a new specific code QUOTA_EXCEEDED for this error scenario, as the default one, TOO_MANY_REQUESTS, is misleading.

403 would be acceptable as well.